A spring core as described above is described in DE-OS No. 3,321,991 of the same applicant. There, however, the opposite legs of the U-shaped end spirals are made of equal length. During automatic assembly of the spring core, adjacent coil springs are joined together at their end spirals along contiguous legs by helical connecting springs. In the assembly of coil springs, according to the subject of DE-OS No. 3,321,991, it was found that an offset of the coil springs will result, that is, without corrective measures in the automatic assembling machine the spring core, whose outer contours are rectangular or square, would be distorted to a rhombus. The coil springs, arranged in rows and columns, thus do not form an angle of 90.degree., but an angle differing therefrom, which on the whole would lead to rhombic external dimensions of the spring core unless appropriate corrective measures are taken in the automatic assembling machine. This, however, involves increased expense.